1. Field of the Invention
The present invention relates to an X-ray image detecting apparatus for detecting an X-ray image, particularly a medical X-ray image, to convert it into a digital image, and an image reading apparatus for reading a general image to digitize it.
2. Related Background Art
Recently, attempt have actively been made to digitize a medical image, particularly an X-ray fluoroscopic image, and to apply it to an electronic filing or automated diagnosis system.
A known example of a device for digitizing a X-ray image is a X-ray image detecting apparatus which is so arranged that X-rays transmitted by a subject are guided onto a fluorescent screen, a storage-type solid state image sensing device such as a CCD receives fluorescence emitted in proportion to X-ray intensity from the fluorescent screen to convert it into video signals, and thereafter the video signals are converted to digital values. There are other apparatus using a device for directly converting X-ray amounts into charge amounts without utilizing fluorescence. Also preferably utilized is a film digitizer which optically reads an image on an X-ray film taken and developed and which converts it into a digital image.
Generally, digital images used in medical diagnosis require delicate density information, and therefore, digital values used therefor are of 12 to 16 bits corresponding to gradients of 4096 to 65536.
However, the digital medical images often have distributions of gradients not effectively distributed over the entire range of the above digital values, but rather are concentrated in a part of the range. If such digital images are displayed on a display apparatus, for example, such as a CRT, images with low contrast are displayed because the display apparatus itself has a lower capability of gradation expression than films.
Thus, the X-ray image detecting apparatus is arranged to output obtained digital data to an external arithmetic unit, to produce a histogram of densities in the arithmetic unit, normally to measure a distribution of frequencies of the image data, to detect an effective range of digital values of the image data, and to produce a data conversion table for expanding the effective range to the entire range of the digital values, that is, to produce a look-up table. To display high-contrast images becomes possible by converting the digital values of image data referring to such a look-up table.
The goal is to automate diagnosis with medical images in order to increase efficiency and accuracy of mass health screening etc., and thus, the histogram is one of the most important information tools in image diagnosis.
Normally, a medical image is composed of a large quantity of pixels, about 2000.times.3000 (6 millions). The conventional example as described above is, however, arranged to produce the histogram by outputting the digital data pixel by pixel from the X-ray image detecting apparatus to the external arithmetic unit, which takes a significant amount of time, and thus greatly lowers the operation efficiency when handling many images.